Characterization of juvenile pyroclasts from the Kos Plateau Tuff (Aegean Arc): insights into the eruptive dynamics of a large rhyolitic eruption
Characterization of juvenile pyroclasts from the Kos Plateau Tuff (Aegean Arc): insights into the eruptive dynamics of a large rhyolitic eruption
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科斯高原凝灰岩（爱琴海弧）幼年火山碎屑的特征：深入了解大型流纹岩喷发的喷发动力学
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发表时间：
2009
期刊：
影响因子：
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通讯作者：
A. Burgisser
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作者：
C. Bouvet de Maisonneuve;O. Bachmann;A. Burgisser
在火山爆发过程中形成的硅质浮石是管道中岩浆状态的忠实记录者，接近或处于破碎水平。我们对161，000年前在希腊东爱琴海弧喷发的无焊接流纹质科斯高原凝灰岩中的四种浮石进行了表征。主要类型的浮石（>90 vol. %）显示了高度细长的管状囊泡。这些管状浮岩出现在整个喷发过程中。不太常见的浮石类型包括：（1）“泡沫”浮石（具有大的，次圆形囊泡的高度多孔），其形成5-10体积%。在粗碎屑流沉积物中，（2）以“微泡”和系统贫结晶浮石为主，见于早期喷发的细粒火山碎屑流单元中，（3）灰色或带状浮岩，表明流纹岩与较镁铁质的岩浆相互作用，它们在整个喷发序列中被发现，并显示出高度不规则的气泡形状。除灰纹浮岩外，其他三种类型的浮岩成分相同，并且是同时生成的，因为它们被发现于同一个火山碎屑岩单元中。因此，它们记录了火山管道中导致可变气泡成核、生长和聚结的不同条件。共74个浮石样品的特点是使用薄片观察，扫描电镜图像，孔隙率测定，渗透率。我们发现，四种浮石类型具有不同的总的和连接的孔隙度，弯曲度和渗透率。灰带状浮石的岩石物理特征变化很大，是两种不同岩浆混合的结果。的微泡，晶体穷人，浮石有双峰气泡尺寸分布，解释为反映早期的非均质气泡成核事件，其次是均匀的气泡成核接近破碎。最后，在孔隙度，弯曲度和渗透性在成分相同的管和泡沫浮石的显着差异是可变的剪切速率在管道的不同部分的结果。不同的剪切速率可能是以下两种情况之一的结果：（1）纯剪切，诱导从泡沫到管状的垂直进展，并意味着一个相对较厚的破碎区，以同时产生两种类型的浮石，或（2）局部简单剪切，诱导强烈的管状囊泡沿着壁和近球形气泡在中心的管道，并不一定需要一个厚的破碎区。
Silicic pumices formed during explosive volcanic eruptions are faithful recorders of the state of the magma in the conduit, close to or at the fragmentation level. We have characterized four types of pumices from the non-welded rhyolitic Kos Plateau Tuff, which erupted 161,000 years ago in the East Aegean Arc, Greece. The dominant type of pumice (>90 vol.%) shows highly elongated tubular vesicles. These tube pumices occur throughout the eruption. Less common pumice types include: (1) “frothy” pumice (highly porous with large, sub-rounded vesicles), which form 5–10 vol.% of the coarsest pyroclastic flow deposits, (2) dominantly “microvesicular” and systematically crystal-poor pumices, which are found in early erupted, fine-grained pyroclastic flow units, and are characterized by many small (<50 μm in diameter) vesicles and few mm-sized, irregular voids, (3) grey or banded pumices, indicating the interaction between the rhyolite and a more mafic magma, which are found throughout the eruption sequence and display highly irregular bubble shapes. Except for the grey-banded pumices, all three other types are compositionally identical and were generated synchronously as they are found in the same pyroclastic units. They, therefore, record different conditions in the volcanic conduit leading to variable bubble nucleation, growth and coalescence. A total of 74 pumice samples have been characterized using thin section observation, SEM imagery, porosimetry, and permeametry. We show that the four pumice types have distinct total and connected porosity, tortuosity and permeability. Grey-banded pumices show large variations in petrophysical characteristics as a response to mingling of two different magmas. The microvesicular, crystal-poor, pumices have a bimodal bubble size distribution, interpreted as reflecting an early heterogeneous bubble nucleation event followed by homogeneous bubble nucleation close to fragmentation. Finally, the significant differences in porosity, tortuosity and permeability in compositionally identical tube and frothy pumices are the result of variable shear rates in different parts of the conduit. Differential shear rates may be the result of either: (1) pure shear, inducing a vertical progression from frothy to tube and implying a relatively thick fragmentation zone to produce both types of pumices at the same time or (2) localized simple shear, inducing strongly tubular vesicles along the wall and near-spherical bubbles in the centre of the conduit and not necessarily requiring a thick fragmentation zone.